The present invention is directed to a deactivatable security label for anti-theft systems wherein a low-coercivity strip can alter an alternating magnetic field generated in a testing zone to thus, trigger an alarm. The low-coercivity strip comprises at least first and second strips arranged close to one another and composed of a ferromagnetic material, at least the first strip being composed of a low-coercivity material having a high permeability and low coercive field strength and at least the second strip having a high coercive field strength and being magnetizable so that the arising scattered field permeates the first strip of low-coercivity material in sections in mutually opposite directions.
An anti-theft system using a strip of the above mentioned type is disclosed in U.S. Pat. No. 3,820,104 whose disclosure is incoporated by reference thereto. As disclosed in this patent, a narrow, low-coercivity strip, which is obtained from a soft-magnetic material, is arranged in the proximity of a significantly broader strip of a magnetic material having a higher coercive field strength. When this arrangement is brought into a zone comprising an alternating magnetic field, then the low-coercivity strip alters this alternating field and induces voltage harmonics in a coil influenced by the alternating field. These voltage harmonics can be interpreted or utilized for triggering an alarm.
When one wishes to deactivate this known security label, then the broader magnetic strip having a high coercive field strength can be magnetized in a longitudinal direction. A fringing flux is generated because of openings which were provided in this broad strip at selected intervals and this fringing flux magnetizes the low-coercivity strip in mutually opposite directions in adjacent sections. In this condition, the low-coercitvity strip is largely saturated so that it does not generate any harmonics or, respectively, generates different and significantly smaller harmonics when it is introduced into the alternating field. Thus, the strip is deactivated and does not trigger an alarm.
Other known arrangement exhibit similar functions. In U.S. Pat. No. 3,747,086, whose disclosure is incorporated by reference thereto, the security labels are composed of two or more strips arranged near one another and having different coercive field strengths. For the deactivation of this label, one of the strips comprises an adequate cross section in order to saturate the low-coercivity strips so that no remagnification of the strip occurs due to the alternating magnetic field.
In another known arrangement, which is disclosed in German OS No. 34 19 785, an amorphous tape is employed as a low-coercivity material. This amorphous tape is connected to short strips of hard magnetic materials which are arranged at intervals from one another. Hereto, the magnetization of these hard magnetic strips generates differently premagnetized sections in the low-coercivity tape so that the low-coercivity tape or strip no longer generates any harmonics which lead to the triggering of an alarm.